Swine influenza virus

Swine influenza (SI) is a respiratory disease caused by infection with the type A swine influenza virus. It is primarily characterized by sudden onset, difficulty breathing, persistent coughing, significant fever, and rapid progression. Typically, outbreaks occur within pig populations, making it highly contagious—but usually not fatal. As a zoonotic disease, swine influenza can severely impact both pig health and the swine industry, while also posing a potential threat to human health. Now, let’s delve into the swine influenza virus itself, exploring its epidemiology, clinical symptoms, associated risks, and detection methods.

Swine influenza is an acute, highly contagious infectious disease caused by swine influenza virus (SIV). SIV belongs to the Orthomyxoviridae family and the Influenza virus genus; its viral particles are spherical in shape, measuring 80–120 nm in diameter. The virus is enveloped, with the envelope consisting of a bilayer lipid membrane, radially arranged glycoprotein spikes, and matrix proteins. SIV is composed of approximately 68%–70% protein, 1%–2% RNA, 20%–25% lipids, and 5%–8% carbohydrates. The viral proteins include five distinct polypeptides: hemagglutinin (HA), neuraminidase (NA), matrix protein (MA), nucleoprotein (NP), and polymerase. The viral genome consists of a single-stranded negative-sense RNA, with a total length of 13.6 kb, organized into eight independent segments of varying sizes. When swine influenza occurs, the most commonly isolated viral subtypes are H1N1 and H3N2.

Swine influenza exhibits a clear seasonal pattern, with outbreaks commonly occurring in early spring, late autumn, and during the cold winter months. Stress factors such as damp and rainy weather, high humidity, crowded conditions, transportation challenges, infestations by internal and external parasites, and malnutrition can all trigger or exacerbate the disease. Pigs of all ages, genders, and breeds are susceptible to infection, which often strikes suddenly. The incubation period typically ranges from 1 to 7 days, rapidly spreading throughout the entire herd. While the incidence rate can reach up to 100%, the mortality rate usually remains below 1%. Swine influenza is a common ailment in large-scale pig farms, frequently manifesting as either localized outbreaks or widespread epidemics. The swine influenza virus persists in the nasal mucosa of infected pigs for up to 8 days post-infection; viral nucleic acids can still be detected in anal swabs from 2 to 8 days after infection. By the third day post-infection, viral nucleic acids become detectable in the larynx and trachea, persisting until around the 10th day. In lung-associated lymph nodes and the spleen, viral nucleic acids test positive between 5 and 7 days post-infection. Notably, no viral nucleic acids were found in other organs, indicating that the respiratory system and spleen are the primary sites where the virus establishes itself. After infection, pigs shed the virus externally for approximately one week. Infected pigs and carrier pigs serve as the main sources of transmission for swine influenza. The virus is present in nasal secretions, tracheal and bronchial exudates, and lung tissues of sick pigs, spreading primarily through respiratory droplets. Even after recovering from the illness, infected pigs may continue to carry the virus for another 6 to 8 weeks.

The incidence of this disease is relatively high, and it typically has a 2- to 7-day incubation period, with the illness usually lasting around 7 days. In the early stages of infection, affected pigs show poor mental condition and a significant increase in body temperature, often reaching 40.2–41.2°C. Their appetite markedly declines, accompanied by excessive discharge from the eyes, along with congestion of the conjunctiva and persistent tearing. Respiratory rates accelerate, sometimes even leading to open-mouth breathing, occasional sneezing, coughing, and photophobia. In some cases, white foamy saliva may appear at the corners of the mouth. Additionally, certain pigs may experience muscle and joint pain, becoming unusually sensitive to external stimuli. Meanwhile, their feces become dry and hard, urine output decreases—or may even cease entirely—and the urine turns dark yellow. The animals also exhibit weakness in their limbs, leaving them unable to stand for extended periods. If effective and appropriate treatment measures are not promptly implemented, the condition of infected pigs may gradually progress into a chronic state. Prolonged coughing and digestive issues will lead to progressive weight loss, increasing the risk of various complications, such as bronchitis, pneumonia, and pleurisy, ultimately driving up mortality rates significantly. Moreover, if sows are infected during pregnancy, the virus can cross the placenta, potentially infecting the developing fetus, resulting in abortion or stillbirths. Even if infected sows recover after treatment, they often give birth to piglets with weakened immunity, stunted growth, and dramatically reduced survival rates. In the absence of complications, the primary gross lesions associated with Swine Influenza (SI) include viral pneumonia, most commonly affecting the apical and cardiac lobes of the lungs. However, in severe cases, nearly the entire lung may be compromised. Hemorrhaging may occur in the mucous membranes of the nose, throat, trachea, and bronchi, which are often filled with blood-tinged fibrinous exudates. Bronchial and mediastinal lymph nodes typically become enlarged, congested, and edematous, while the spleen is usually mildly swollen. Gastrointestinal tissues may also show catarrhal inflammation. When secondary bacterial infections occur, the pathological changes become more complex. Notably, the severity of these pathological alterations is closely tied to the specific strain of the virus responsible for the outbreak.

Infection with swine influenza can lead to lesions in the respiratory, lymphatic, and digestive systems, thereby impacting research on specific diseases and their corresponding therapeutic drugs. Since pigs possess sialic acid receptors in their respiratory tracts that are receptive to both human and avian influenza viruses, they serve as a common susceptible host for avian, swine, and human influenza viruses—and act as a "mixing vessel" where genetic recombination or reassortment of influenza viruses can occur. As a result, the public health significance of swine influenza is increasingly prominent, playing a critical role during influenza pandemics. In 2015, swine flu struck India, infecting 31,000 people and claiming over 1,800 lives. By early 2016, more than 200 people had already died from influenza in Russia, with most fatalities confirmed to be infected with the H1N1 swine influenza virus.

The main diagnostic methods for SIV include viral isolation and identification, serological techniques such as the HI test, latex agglutination assay, and ELISA, as well as nucleic acid-based diagnostic methods like conventional PCR and quantitative PCR. Viral isolation and identification are time-consuming and often less accurate, so they are now used less frequently. Meanwhile, with the increasing number of SIV serum subtypes and the continuous emergence of new variant strains, the application of serological methods like ELISA has become somewhat limited. Therefore, PCR-based detection methods are typically employed in laboratory settings for diagnosing swine influenza virus.

The primary measures are strict biosecurity and vaccine immunization. Since cross-species transmission is possible, pigs should be kept away from other animal species, especially poultry. Individuals suspected of influenza virus infection should also avoid contact with pigs. Currently, the vaccines available on the market mainly include inactivated vaccines and subunit vaccines containing H1N1 and H3N2 strains; however, China has yet to develop a commercially available swine influenza vaccine. Commercially available swine flu vaccines typically consist of inactivated whole-virus formulations, which have played a crucial role in preventing swine influenza. Yet, these inactivated vaccines can complicate the differential diagnosis between vaccinated pigs and those naturally infected, thereby hindering disease surveillance efforts. Moreover, inactivated vaccines fail to provide effective protection against heterologous or heterotypic viral strains. As a result, it remains challenging for inactivated vaccines to effectively control the infection and transmission of the highly antigenically variable swine influenza virus within pig populations. On the other hand, live attenuated swine influenza virus vaccines could offer cross-protection among different viral strains—but so far, safe and viable attenuated vaccine strains have not yet been successfully developed.

According to a report from China National Radio Network, Beijing, July 9, 2022, the Chinese Center for Disease Control and Prevention has revealed that the H9N2 avian influenza virus is currently infecting pigs in China and undergoing genetic recombination with swine influenza viruses. This emerging strain poses a potential risk of zoonotic transmission. As the flu season approaches, what changes should laboratory animal facilities and professionals consider? Readers are encouraged to share your valuable experiences and suggestions.

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